To provide a device capable of quickly indexing a passing core of a subject.

This centering device is provided with first and second centering mechanisms 8 and 9 positioned at an interval on a cylindrical inside surface of a nozzle 2, and respectively extensible in the radial direction, a concentric shaft 10 for coaxially supporting these first and second centering mechanisms 8 and 9, handles 14a and 14b for extending the first and second centering mechanism 8 and 9 in the radial direction by rotation from an external part of the nozzle 2, and a ruled line writing needle 16 installed on the concentric shaft 10, and marking a work ruled line c for regulating a surface at a right angle to the axis of the nozzle 2 on the outer periphery of the nozzle 2.